Generally of volcanic origin, the spheres typically occur as accretionary lapilli, which are ash aggregates that form during explosive volcanic eruptions – of which there were many in this part ...

The nature of their solid products, lava, scoria, lapilli, and ashes ... apparatus to collect gases not only at the orifice of the volcano, but at great depths in tht vent; in the latter case ...

The Institute also said that thin volcanic dust can irritate people’s eyes, skin and throats, while crops can suffer irreversible damage due to the ashes and lapilli transported by the wind ...